Plasma lamp with dielectric waveguide

ABSTRACT

A dielectric waveguide integrated plasma lamp is disclosed for powering a small and bright bulb with a diameter of a few millimeters. The lamp is contained within a high dielectric constant material which guides the microwaves to the bulb, provides heat isolation to the drive circuit, contains the microwaves, provides structural stability and ease of manufacturing and allows efficient energy coupling to the bulb when used as a dielectric resonant oscillator.

[0001] This application claims priority to a U.S. ProvisionalApplication entitled “Plasma Lamp,” having Ser. No. 60/222,028 and filedon Jul. 31, 2000, and a U.S. which is hereby incorporated by referenceas though fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The field of the present invention relates to devices and methodsfor generating light, and more particularly to electrodeless plasmalamps.

[0004] 2. Background

[0005] Electrodeless plasma lamps provide point-like, bright, whitelight sources. Because they do not use electrodes, electrodeless plasmalamps often have longer useful lifetimes than other lamps. Electrodelessplasma lamps in the prior art have certain common features. For examplein U.S. Pat. Nos. 4,954,755 to Lynch et al., 4,975,625 to Lynch et al.,4,978,891 to Ury et al., 5,021,704 to Walter et al., 5,448,135 toSimpson, 5,594,303 to Simpson, 5,841,242 to Simpson et al., 5,910,710 toSimpson, and 6,031,333 to Simpson, each of which is incorporated hereinby reference, the plasma lamps direct microwave energy into an aircavity, with the air cavity enclosing a bulb containing a mixture ofsubstances that can ignite, form a plasma, and emit light.

[0006] The plasma lamps described in these patents are intended toprovide brighter light sources with longer life and more stable spectrumthan electrode lamps. However, for many applications, light sources thatare brighter, smaller, less expensive, more reliable, and have longuseful lifetimes are desired, but such light sources until now have beenunavailable. Such applications include, for example, streetlights andemergency response vehicles. A need exists therefore, for a very bright,durable light source at low cost.

[0007] In the prior art, the air-filled cavity of the electrodelessplasma lamp is typically constructed in part by a metal mesh. Metal meshis used because it contains the microwave energy within the cavity whileat the same time permitting the maximum amount of visible light toescape. The microwave energy is typically generated by a magnetron orsolid state electronics and is guided into the cavity through one ormore waveguides. Once in the air-filled cavity, microwave energy ofselect frequencies resonates, where the actual frequencies that resonatedepend upon the shape and size of the cavity. Although there istolerance in the frequencies that may be used to power the lamps, inpractice, the power sources are limited to microwave frequencies in therange of 1-10 GHz.

[0008] Because of the need to establish a resonance condition in theairfilled cavity, the cavity generally may not be smaller than one-halfthe wavelength of the microwave energy used to power the lamp. Theair-filled cavity and thereby, the plasma lamp itself has a lower limiton its size. However, for many applications, such as for high-resolutionmonitors, bright lamps, and projection TVs, these sizes remainprohibitively large. A need exists therefore for a plasma lamp that isnot constrained to the minimum cavity sizes illustrated by the priorart.

[0009] In the prior art, the bulbs are typically positioned at a pointin the cavity where the electric field created by the microwave energyis at a maximum. The support structure for the bulb is preferably of asize and composition that does not interfere with the resonatingmicrowaves, as any interference with the microwaves reduces theefficiency of the lamp. The bulbs, therefore, are typically made fromquartz. Quartz bulbs, however, are prone to failure because the plasmatemperature can be several thousand degrees centigrade, which can bringthe quartz wall temperature to near 1000° C. Furthermore, quartz bulbsare unstable in terms of mechanical stability and optical and electricalproperties over long periods. A need exists, therefore, for a lightsource that overcomes the above-described issues, but that is alsostable in its spectral characteristics over long periods.

[0010] In prior art plasma lamps, the bulb typically contains a noblegas combined with a light emitter, a second element or compound whichtypically comprises sulfur, selenium, a compound containing sulfur orselenium, or any one of a number of metal halides. Exposing the contentsof the bulb to microwave energy of high intensity causes the noble gasto become a plasma. The free electrons within the plasma excite thelight emitter within the bulb. When the light emitter returns to a lowerelectron state, radiation is emitted. The spectrum of light emitteddepends upon the characteristics of the light emitter within the bulb.Typically, the light emitter is chosen to cause emission of visiblelight.

[0011] Plasma lamps of the type described above frequently require highintensity microwaves to initially ignite the noble gas into plasma.However, over half of the energy used to generate and maintain theplasma is typically lost as heat, making heat dissipation a problem. Hotspots can form on the bulb causing spotting on the bulb and therebyreducing the efficiency of the lamp. Methods have been proposed toreduce the hot spots by rotating the lamp to better distribute theplasma within the lamp and by blowing constant streams of air at thelamp. These solutions, however, add structure to the lamp, therebyincreasing its size and cost. Therefore, a need exists for a plasma lampthat requires less energy to ignite and maintain the plasma, andincludes a minimum amount of additional structure for efficientdissipation of heat.

SUMMARY OF THE INVENTION

[0012] This invention generally provides, in one aspect, devices andmethods of producing bright, spectrally stable light.

[0013] In accordance with one embodiment as described herein, a devicefor producing light comprises an electromagnetic energy source, awaveguide having a body formed of a dielectric material, and a bulb.Preferably, the waveguide is connected to the energy source forreceiving electromagnetic energy from the energy source. The waveguidebuilds and contains the electromagnetic energy. The bulb, which iscoupled to the waveguide, receives electromagnetic energy from thewaveguide. The received electromagnetic energy ignites a gas-fill thatforms a plasma and emits light, preferably in the visible spectralrange.

[0014] In one preferred embodiment, the bulb is shaped to reflect lightoutwards through its window. The electromagnetic energy source ispreferably a microwave energy source that is efficiently coupled to andpreferably thermally isolated from the waveguide. Furthermore, the outersurface of the waveguide, preferably with the exception of the bulbcavity, is coated with a material to contain the microwave energy withinthe waveguide. The dielectric forming the waveguide preferably has ahigh dielectric constant, a high dielectric strength, and a low losstangent. This permits high power densities within the waveguide. A heatsink preferably is attached to the outer surfaces of the waveguide todissipate heat.

[0015] In accordance with a first alternative embodiment, the lamp isoperated in resonant cavity mode. In this mode, the microwave energydirected into the waveguide has a frequency such that it resonateswithin the waveguide. The microwave feed and the bulb are preferablypositioned at locations with respect to the waveguide that correspond toelectric field maxima of the resonant frequency.

[0016] In accordance with a second alternative embodiment, the lamp isoperated in a dielectric oscillator mode. In this mode, an energyfeedback mechanism or probe is coupled to the dielectric waveguide at apoint that in one embodiment corresponds to an energy maximum. The probesenses the electric field amplitude and phase within the waveguide atthe point of coupling. Using the probe signal to provide feedback, thelamp may be continuously operated in resonant cavity mode, even if theresonant frequency changes as the plasma forms in the bulb and/or if thedielectric waveguide undergoes thermal expansion due to the heatgenerated. The probe provides feedback to the microwave source and themicrowave source adjusts its output frequency to dynamically maintain aresonance state.

[0017] Further embodiments, variations and enhancements, includingcombinations of the above-described embodiments, or features thereof,are also described herein or depicted in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates a sectional view of a plasma lamp according toa preferred embodiment.

[0019]FIGS. 2A and 2B illustrate sectional views of alternativeembodiments of a plasma lamp.

[0020]FIGS. 3A and 3B illustrate a sectional view of an alternativeembodiment of a plasma lamp wherein the bulb is thermally isolated fromthe dielectric waveguide.

[0021] FIGS. 4A-D illustrate different resonant modes within arectangular prism-shaped waveguide.

[0022] FIGS. 5A-C illustrate different resonant modes within using acylindrical prism-shaped cylindrical waveguide.

[0023]FIG. 6 illustrates an embodiment of the apparatus using a feedbackmechanism to provide feedback to the microwave source to maintain aresonant mode of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Turning now to the drawings, FIG. 1 illustrates a preferredembodiment of a dielectric waveguide integrated plasma lamp 101 (DWIPL).The DWIPL 101 preferably comprises a source 115 of electromagneticradiation, preferably microwave radiation, a waveguide 103 having a bodyformed of a dielectric material, and a feed 117 coupling the radiationsource 115 to the waveguide 103. As used herein, the term “waveguide”generally refers to any device having a characteristic and purpose of atleast partially confining electromagnetic energy. The DWIPL 101 furtherincludes a bulb 107, that is preferably disposed on an opposing side ofthe waveguide 103, and contains a gas-fill, preferably comprising anoble gas and a light emitter, which when receiving electromagneticenergy at a specific frequency and intensity forms a plasma and emitslight.

[0025] In a preferred embodiment, the microwave radiation source 115feeds the waveguide 103 microwave energy via the feed 117. The waveguidecontains and guides the microwave energy to a cavity 105 preferablylocated on an opposing side of the waveguide 103 from the feed 117.Disposed within the cavity 105 is the bulb 107 containing the gas-fill.Microwave energy is preferably directed into the enclosed cavity 105,and in turn the bulb 107. This microwave energy generally freeselectrons from their normal state and thereby transforms the noble gasinto a plasma. The free electrons of the noble gas excite the lightemitter. The de-excitation of the light emitter results in the emissionof light. As will become apparent, the different embodiments of DWIPLsdisclosed herein offer distinct advantages over the plasma lamps in theprior art, such as an ability to produce brighter and spectrally morestable light, greater energy efficiency, smaller overall lamp sizes, andlonger useful life spans.

[0026] The microwave source 115 in FIG. I is shown schematically assolid state electronics, however, other devices commonly known in theart that can operate in the 0.5-30 GHz range may also be used as amicrowave source, including but not limited to klystrons and magnetrons.The preferred range for the microwave source is from about 500 MHz toabout 10 GHz.

[0027] Depending upon the heat sensitivity of the microwave source 115,the microwave source 115 may be thermally isolated from the bulb 107,which during operation preferably reaches temperatures between about700° C. and about 1000° C. Thermal isolation of the bulb 107 from thesource 115 provides a benefit of avoiding degradation of the source 115.Additional thermal isolation of the microwave source 115 may beaccomplished by any one of a number of methods commonly known in theart, including but not limited to using an insulating material or vacuumgap occupying an optional space 116 between the source 115 and waveguide103. If the latter option is chosen, appropriate microwave feeds areused to couple the microwave source 115 to the waveguide 103.

[0028] In FIG. 1, the feed 117 that transports microwaves from thesource 115 to the waveguide 103 preferably comprises a coaxial probe.However, any one of several different types of microwave feeds commonlyknown in the art may be used, such as microstrip lines or fin linestructures.

[0029] Due to mechanical and other considerations such as heat,vibration, aging, or shock, when feeding microwave signals into adielectric material, contact between the feed 117 and the waveguide 103is preferably maintained using a positive contact mechanism 121. Thecontact mechanism 121 provides constant pressure between the feed 117and the waveguide 103 to minimize the probability that microwave energywill be reflected back through the feed 117 and not transmitted into thewaveguide 103. In providing constant pressure, the contact mechanism 121compensates for small dimensional changes in the microwave feed 117 andthe waveguide 103 that may occur due to thermal heating or mechanicalshock. The contact mechanism may be a spring loaded device, such as isillustrated in FIG. 1, a bellows type device, or any other devicecommonly known in the art that can sustain a constant pressure forcontinuously and steadily transferring microwave energy.

[0030] When coupling the feed 117 to the waveguide 103, intimate contactis preferably made by depositing a metallic material 123 directly on thewaveguide 103 at its point of contact with the feed 117. The metallicmaterial 123 eliminates gaps that may disturb the coupling and ispreferably comprised of gold, silver, or platinum, although otherconductive materials may also be used. The metallic material 123 may bedeposited using any one of several methods commonly known in the art,such as depositing the metallic material 123 as a liquid and then firingit in an oven to provide a solid contact.

[0031] In FIG. 1, the waveguide 103 is preferably the shape of arectangular prism, however, the waveguide 103 may also have acylindrical prism shape, a sphere-like shape, or any other shape,including a complex, irregular shape the resonant frequencies of whichare preferably determined through electromagnetic simulation tools, thatcan efficiently guide microwave energy from the feed 117 to the bulb107. The actual dimensions of the waveguide may vary depending upon thefrequency of the microwave energy used and the dielectric constant ofthe body of waveguide 103.

[0032] In one preferred embodiment, the waveguide body is approximately12,500 mm³ with a dielectric constant of approximately 9 and operatingfrequency of approximately 2.4 GHz. Waveguide bodies on this scale aresignificantly smaller than the waveguides in the plasma lamps of theprior art. As such, the waveguides in the preferred embodimentsrepresent a significant advance over the prior art because the smallersize allows the waveguide to be used I many applications, wherewaveguide size had previously prohibited such use or made such usewholly impractical. For larger dielectric constants, even smaller sizesfor the waveguides may be achieved. Besides the obvious advantagescreated by a reduction in size, size reduction translates into a higherpower density, lower loss, and thereby, an ease in igniting the lamp.

[0033] Regardless of its shape and size, the waveguide 103 preferablyhas a body comprising a dielectric material which, for example,preferably exhibits the following properties: (1) a dielectric constantpreferably greater than approximately 2; (2) a loss tangent preferablyless than approximately 0.01; (3) a thermal shock resistance quantifiedby a failure temperature of preferably greater than approximately 200°C.; (4) a DC breakdown threshold of preferably greater thanapproximately 200 kilovolts/inch; (5) a coefficient of thermal expansionof preferably less than approximately 10⁻⁵/° C.; (6) a zero or slightlynegative temperature coefficient of the dielectric constant; (7)stoichemetric stability over a preferred range of temperature,preferably from about −80° C. to about 1000° C., and (8) a thermalconductivity of preferably approximately 2 W/mK (watts per milliKelvin).

[0034] Certain ceramics, including alumina, zirconia, titanates, andvariants or combinations of these materials, and silicone oil maysatisfy many of the above preferences, and may be used because of theirelectrical and thermo-mechanical properties. In any event, it should benoted that the embodiments presented herein are not limited to awaveguide exhibiting all or even most of the foregoing properties.

[0035] In the various embodiments of the waveguide disclosed herein,such as in the example outlined above, the waveguide preferably providesa substantial thermal mass, which aids efficient distribution anddissipation of heat and provides thermal isolation between the lamp andthe microwave source.

[0036] Alternative embodiments of DWIPLS 200, 220 are depicted in FIGS.2A-B. In FIG. 2A, a bulb 207 and bulb cavity 205 are provided on oneside of a waveguide 203, preferably on a side opposite a feed 209, andmore preferably in the same plane as the feed 209, where the electricfield of the microwave energy is at a maximum. Where more than onemaximum of the electric field is provided in the waveguide 203, the bulb207 and bulb cavity 205 may be positioned at one maximum and the feed209 at another maximum. By placing the feed 209 and bulb 207 at amaximum for the electric field, a maximum amount of energy isrespectively transferred and intercepted. The bulb cavity 205 is aconcave form in the body of the waveguide 203.

[0037] As shown in FIG. 2B, the body of the waveguide 223 optionallyprotrudes outwards in a convex form, from the main part of the body ofthe waveguide 203 to form the bulb cavity 225. As in FIG. 2A, in FIG.2B, the bulb 227 is preferably positioned opposite to the feed 221.However, where more than one electric field maximum is provided in thewaveguide 203, the bulb 207, 227 may be positioned in a plane other thanthe plane of the feed 209, 221.

[0038] Returning to FIG. 1, the outer surfaces of the waveguide 103,with the exception of those surfaces forming the bulb cavity 105, arepreferably coated with a thin metallic coating 119 to reflect themicrowaves. The overall reflectivity of the coating 119 determines thelevel of energy contained within the waveguide 103. The more energy thatcan be stored within the waveguide 103, the greater the overallefficiency of the lamp 101. The coating 119 also preferably suppressesevanescent radiation leakage. In general, the coating 119 preferablysignificantly eliminates any stray microwave field.

[0039] Microwave leakage from the bulb cavity 105 may be significantlyattenuated by having a cavity 105 that is preferably significantlysmaller than the microwave wavelengths used to operate the lamp 101. Forexample, the length of the diagonal for the window is preferablyconsiderably less than half of the microwave wavelength (in free space)used.

[0040] The bulb 107 is disposed within the bulb cavity 105, andpreferably comprises an outer wall 109 and a window 111. In onepreferred embodiment, the cavity wall of the body of the waveguide 103acts as the outer wall of the bulb 107. The components of the bulb 107preferably include one or more dielectric materials, such as ceramicsand sapphires. In one embodiment, the ceramics in the bulb are the sameas the material used in waveguide 103. Dielectric materials arepreferred for the bulb 107 because the bulb 107 is preferably surroundedby the dielectric body of the waveguide 103 and the dielectric materialshelp ensure efficient coupling of the microwave energy with the gas-fillin the bulb 107.

[0041] The outer wall 109 is preferably coupled to the window 111 usinga seal 113, thereby defining a bulb envelope 127 which contains thegas-fill comprising the plasma-forming gas and light emitter. Theplasma-forming gas is preferably a noble gas, which enables theformation of a plasma. The light emitter is preferably a vapor formed ofany one of a number of elements or compounds currently known in the art,such as sulfur, selenium, a compound containing sulfur or selenium, orany one of a number of metal halides, such as indium bromide (InBr₃).

[0042] To assist in confining the gas-fill within the bulb 107, the seal113 preferably comprises a hermetic seal. The outer wall 109 preferablycomprises alumina because of its white color, temperature stability, lowporosity, and thermal expansion coefficient. However, other materialsthat generally provide one or more of these properties may be used. Theouter wall 109 is also preferably contoured to reflect a maximum amountof light out of the cavity 105 through the window 111. For instance, theouter wall 109 may have a parabolic contour to reflect light generatedin the bulb 107 out through the window 111. However, other outer wallcontours or configurations that facilitate directing light out throughthe window 111 may be used.

[0043] The window 111 preferably comprises sapphire for lighttransmittance and because its thermal expansion coefficient matches wellwith alumina. Other materials that have a similar light transmittanceand thermal expansion coefficient may be used for the window 111. In analternative embodiment, the window 111 may comprise a lens to collectthe emitted light.

[0044] As referenced above, during operation, the bulb 107 may reachtemperatures of up to about 1000° C. Under such conditions, thewaveguide 103 in one embodiment acts as a heat sink for the bulb 107. Byreducing the heat load and heat-induced stress upon the variouscomponents of the DWIPL 101, the useful life span of the DWIPL 101 isgenerally increased beyond the life span of typical electrodeless lamps.Effective heat dissipation may be obtained by preferably placingheat-sinking fins 125 around the outer surfaces of the waveguide 103, asdepicted in FIG. 1. In the embodiment shown in FIG. 2B, with the cavity225 extending away from the main part of the body of the waveguide 223,the DWIPL 220 may be used advantageously to remove heat more efficientlyby placing fins 222 in closer proximity to the bulb 227.

[0045] In another embodiment, the body of the waveguide 103 comprises adielectric, such as a titanate, which is generally not stable at hightemperatures. In this embodiment, the waveguide 103 is preferablyshielded from the heat generated in the bulb 107 by placing a thermalbarrier between the body of the waveguide 103 and the bulb 107. In onealternative embodiment, the outer wall 109 acts as a thermal barrier bycomprising a material with low thermal conductivity such as NZP. Othersuitable material for a thermal barrier may also be used.

[0046]FIGS. 3A and 3B illustrate an alternative embodiment of a DWIPL300 wherein a vacuum gap acts as a thermal barrier. As shown in FIG. 3A,the bulb 313 of the DWIPL 300 is disposed within a bulb cavity 315 andis separated from the waveguide 311 by a gap 317, the thickness of whichpreferably varies depending upon the microwave propagationcharacteristics and material strength of the material used for the bodyof the waveguide 311 and the bulb 313. The gap 317 is preferably avacuum, minimizing heat transfer between the bulb 313 and the waveguide311.

[0047]FIG. 3B illustrates a magnified view of the bulb 313, bulb cavity315, and vacuum gap 317 for the DWIPL 300. The boundaries of the vacuumgap 317 are formed by the waveguide 311, a bulb support 319, and thebulb 313. The bulb support 319 may be sealed to the waveguide 311, thesupport 319 extending over the edges of the bulb cavity 315 andcomprising a material such as alumina that preferably has high thermalconductivity to help dissipate heat from the bulb 313.

[0048] Embedded in the support 319 is an access seal 321 forestablishing a vacuum within the gap 317 when the bulb 313 is in place.The bulb 313 is preferably supported by and hermetically sealed to thebulb support 319. Once a vacuum is established in the gap 317, heattransfers between the bulb 313 and the waveguide 311 are preferablysubstantially reduced.

[0049] Embodiments of the DWIPLs thus far described preferably operateat a microwave frequency in the range of 0.5-10 GHz. The operatingfrequency preferably excites one or more resonant modes supported by thesize and shape of the waveguide, thereby establishing one or moreelectric field maxima within the waveguide. When used as a resonantcavity, at least one dimension of the waveguide is preferably an integernumber of half-wavelengths long.

[0050] FIGS. 4A-C illustrate three alternative embodiments of DWIPLs410, 420, 430 operating in different resonant modes. FIG. 4A illustratesa DWIPL 410 operating in a first resonant mode 411 where one axis of arectangular prism-shaped waveguide 417 has a length that is one-half thewavelength of the microwave energy used. FIG. 4B illustrates a DWIPL 420operating in a resonant mode 421 where one axis of a rectangularprism-shaped waveguide 427 has a length that is equal to one wavelengthof the microwave energy used. FIG. 4C illustrates a DWIPL 430 operatingin a resonant mode 431 where one axis of a rectangular prism-shapedwaveguide 437 has a length that is 1½ wavelengths of the microwaveenergy used.

[0051] In each of the DWIPLs and corresponding modes depicted in FIGS.4A-C, and for DWIPLs operating at any higher modes, the bulb cavity 415,425, 435 and the feed(s) 413, 423, 433, 434 are preferably positionedwith respect to the waveguide 417, 427, 437 at locations where theelectric fields are at an operational maximum. However, the bulb cavityand the feed do not necessarily have to lie in the same plane.

[0052]FIG. 4C illustrates an additional embodiment of a DWIPL 430wherein two feeds 433, 434 are used to supply energy to the waveguide437. The two feeds 433, 434 may be coupled to a single microwave sourceor multiple sources (not shown).

[0053]FIG. 4D illustrates another embodiment wherein a single energyfeed 443 supplies energy into the waveguide 447 having multiple bulbcavities 415, 416, each positioned with respect to the waveguide 447 atlocations where the electric field is at a maximum.

[0054] FIGS. 5A-C illustrate DWIPLs 510, 520, 530 having cylindricalprism-shaped waveguides 517, 527, 537. In the embodiments depicted inFIGS. 5A-C, the height of the cylinder is preferably less than itsdiameter, the diameter preferably being close to an integer multiple ofthe lowest order half-wavelength of energy that can resonate within thewaveguide 517, 527, 537. Placing such a dimensional restriction on thecylinder results in the lowest resonant mode being independent of theheight of the cylinder. The diameter of the cylinder thereby dictatesthe fundamental mode of the energy within the waveguide 517, 527, 537.The height of the cylinder can therefore be optimized for otherrequirements such as size and heat dissipation. In FIG. 5A, the feed 513is preferably positioned directly opposite the bulb cavity 515 and thezeroeth order Bessel mode 511 is preferably excited.

[0055] Other modes may also be excited within a cylindrical prism-shapedwaveguide. For example, FIG. 5B illustrates a DWIPL 520 operating in aresonant mode where the cylinder 527 has a diameter that is preferablyclose to one wavelength of the microwave energy used.

[0056] As another example, FIG. 5C illustrates a DWIPL 520 operating ina resonant mode where the cylinder 537 has a diameter that is preferablyclose to ½ wavelengths of the microwave energy used. FIG. 5Cadditionally illustrates an embodiment of a DWIPL 530 whereby two feeds533, 534 are used to supply energy to the cylinder-shaped waveguide 537.As with other embodiments of the DWIPL, in a DWIPL having acylinder-shaped waveguide, the bulb cavity 515, 525, 535 and the feed(s)513, 523, 533, 534 are preferably positioned with respect to thewaveguide 517, 527, 537 at locations where the electric field is at amaximum.

[0057] Using a dielectric waveguide has several distinct advantages.First, as discussed above, the waveguide may be used to help dissipatethe heat generated in the bulb. Second, higher power densities may beachieved within a dielectric waveguide than are possible in the plasmalamps with air cavities that are currently used in the art. The energydensity of a dielectric waveguide is greater, depending on thedielectric constant of the material used for the waveguide, than theenergy density of an air cavity plasma lamp.

[0058] Referring back to the DWIPL 101 of FIG. 1, high resonant energywithin the waveguide 103, corresponding to a high value for Q (where Qis the ratio of the operating frequency to the frequency width of theresonance) for the waveguide results in a high evanescent leakage ofmicrowave energy into the bulb cavity 105. High leakage in the bulbcavity 105 leads to the quasi-static breakdown of the noble gas withinthe envelope 127, thus generating the first free electrons. Theoscillating energy of the free electrons scales as ¦λ², where λ is thecirculating intensity of the microwave energy and λ is the wavelength ofthat energy. Therefore, the higher the microwave energy, the greater isthe oscillating energy of the free electrons. By making the oscillatingenergy greater than the ionization potential of the gas,electron-neutral collisions result in efficient build-up of plasmadensity.

[0059] Once the plasma is formed in the DWIPL and the incoming power isabsorbed, the waveguide's Q value drops due to the conductivity andabsorption properties of the plasma. The drop in the Q value isgenerally due to a change in the impedance of the waveguide. Afterplasma formation, the presence of the plasma in the cavity makes thebulb cavity absorptive to the resonant energy, thus changing the overallimpedance of the waveguide. This change in impedance is effectively areduction in the overall reflectivity of the waveguide. Therefore, bymatching the reflectivity of the feed close to the reduced reflectivityof the waveguide, a sufficiently high Q value may be obtained even afterthe plasma formation to sustain the plasma. Consequently, a relativelylow net reflection back into the energy source may be realized.

[0060] Much of the energy absorbed by the plasma eventually appears asheat, such that the temperature of the lamp may approach 1000° C. Whenthe waveguide is also used as a heat sink, as previously described, thedimensions of the waveguide may change due to its coefficient of thermalexpansion. Under such circumstances, when the waveguide expands, themicrowave frequency that resonates within the waveguide changes andresonance is lost. In order for resonance to be maintained, thewaveguide preferably has at least one dimension equal to an integermultiple of the half wavelength microwave frequency being generated bythe microwave source.

[0061] One preferred embodiment of a DWIPL that compensates for thischange in dimensions employs a waveguide comprising a dielectricmaterial having a temperature coefficient for the refractive index thatis approximately equal and opposite in sign to its temperaturecoefficient for thermal expansion. Using such a material, a change indimensions due to thermal heating offsets the change in refractiveindex, minimizing the potential that the resonant mode of the cavitywould be interrupted. Such materials include Titanates. A secondembodiment that compensates for dimensional changes due to heatcomprises physically tapering the walls of the waveguide in apredetermined manner.

[0062] In another preferred embodiment, schematically shown in FIG. 6, aDWIPL 610 may be operated in a dielectric resonant oscillator mode. Inthis mode, first and second microwave feeds 613, 615 are coupled betweenthe dielectric waveguide 611, which may be of any shape previouslydiscussed, and the microwave energy source 617. The energy source 617 ispreferably broadband with a high gain and high power output and capableof driving plasma to emission.

[0063] The first feed 613 may generally operate as described above inother embodiments. The second feed 615 may probe the waveguide 611 tosample the field (including the amplitude and phase informationcontained therein) present and provide its sample as feedback to aninput of the energy source 617 or amplifier. In probing the waveguide611, the second feed 615 also preferably acts to filter out strayfrequencies, leaving only the resonant frequency within the waveguide611.

[0064] In this embodiment, the first feed 613, second feed, 615 and bulbcavity 619 are each preferably positioned with respect to the waveguide611 at locations where the electric field is at a maximum. Using thesecond feed 615, the energy source 617 amplifies the resonant energywithin the waveguide 611. The source 617 thereby adjusts the frequencyof its output to maintain one or more resonant modes in the waveguide611. The complete configuration thus forms a resonant oscillator. Inthis manner, automatic compensation may be realized for frequency shiftsdue to plasma formation and thermal changes in dimension and thedielectric constant.

[0065] The dielectric resonant oscillator mode also enables the DWIPL610 to have an immediate re-strike capability after being turned off. Aspreviously discussed, the resonant frequency of the waveguide 611 maychange due to thermal expansion or changes in the dielectric constantcaused by heat generated during operation. When the DWIPL 610 isshutdown, heat is slowly dissipated, causing instantaneous changes inthe resonant frequency of the waveguide 611.

[0066] However, as indicated above, in the resonant oscillator mode theenergy source 617 automatically compensates for changes in the resonantfrequency of the waveguide 611. Therefore, regardless of the startupcharacteristics of the waveguide 611, and providing that the energysource 617 has the requisite bandwidth, the energy source 617 willautomatically compensate to achieve resonance within the waveguide 611.The energy source immediately provides power to the DWIPL at the optimumplasma-forming frequency.

[0067] While embodiments and advantages of this invention have beenshown and described, it would be apparent to those skilled in the artthat many more modifications are possible without departing from theinventive concepts herein. The invention, therefore, is not to berestricted except in the spirit of the appended claims.

What is claimed is:
 1. A lamp comprising: (a) a waveguide having a bodycomprising a dielectric material, said waveguide configured to beconnected to an energy source for receiving electromagnetic energy; and(b) a bulb coupled to the waveguide and containing a gas-fill that emitslight when receiving the electromagnetic energy from the waveguide. 2.The lamp of claim 1, wherein the body of the waveguide includes an outercoating comprising an electrically conductive material.
 3. The lamp ofclaim 1, wherein the bulb comprises a cavity in the body of thewaveguide, and a window coupled to and covering the cavity.
 4. The lampof claim 3, wherein the window is substantially transparent to theemitted light.
 5. The lamp of claim 3, wherein the window is comprisedof sapphire.
 6. The lamp of claim 3, wherein the window comprises afocusing lens.
 7. The lamp of claim 1, wherein the body of the waveguideincludes a cavity, and the bulb is at least in part positioned in thecavity.
 8. The lamp of claim 7, wherein the bulb comprises a ceramicenclosure coupled to a sapphire window.
 9. The lamp of claim 7, whereinthe body of the waveguide includes a main part and a protrusion from themain part, and the cavity is positioned in the protrusion.
 10. The lampof claim 3, wherein the body of the waveguide includes a main part and aprotrusion from the main part, and the cavity is positioned in theprotrusion.
 11. The lamp of claim 1, further comprising a first energyfeed coupled to the waveguide for receiving the electromagnetic energy.12. The lamp of claim 1, wherein the light is visible, infrared, orultra violet-light.
 13. The lamp of claim 1, wherein the dielectricmaterial has a dielectric constant greater than approximately 2.0. 14.The lamp of claim 1, wherein the electromagnetic energy has a frequencybetween about 0.5 and about 10 GHz.
 15. The lamp of claim 1, wherein thewalls of the bulb are at least partially reflective of the light. 16.The lamp of claim 1, wherein the walls of the bulb are shaped to reflectthe light towards the window.
 17. The lamp of claim 1, wherein the wallsof the bulb comprise a dielectric material.
 18. The lamp of claim 1,wherein the dielectric material is a ceramic.
 19. The lamp of claim 1,wherein the walls of the bulb thermally isolate the bulb from thewaveguide.
 20. The lamp of claim 1, wherein the window and the walls ofthe bulb have approximately equal thermal expansion coefficients. 21.The lamp of claim 2, wherein the outer coating of the waveguide isthermally conductive.
 22. The lamp of claim 1, further comprising a heatsink connected to an outer surface of the waveguide.
 23. The lamp ofclaim 1, wherein the waveguide has a rectangular prism-like shape. 24.The lamp of claim 1, wherein the waveguide has cylindrical prism-likeshape.
 25. The lamp of claim 1, wherein the waveguide is sphere-like inshape.
 26. The lamp of claim 1, further comprising an energy feedcoupled to the waveguide for receiving the electromagnetic energy,wherein a positive force mechanism maintains constant contact betweenthe first energy feed and the waveguide.
 27. The lamp of claim 1,wherein the energy source is thermally isolated from the waveguide andthe bulb.
 28. The lamp of claim 1, wherein the gas-fill comprises anoble gas and a metal halide.
 29. The lamp of claim 1, furthercomprising a thermal isolation layer disposed between the bulb and thewaveguide.
 30. The lamp of claim 29, wherein the thermal isolation layercomprises an evacuated space.
 31. The lamp of claim 1, wherein anelectromagnetic field resonates within the waveguide and includes atleast one resonant maximum.
 32. The lamp of claim 31, further comprisinga first energy feed coupled to the waveguide for receiving theelectromagnetic energy, wherein the bulb and the first energy feed areproximate to one of the at least one resonant maximum.
 33. The lamp ofclaim 31, further comprising a first energy feed coupled to thewaveguide for receiving the electromagnetic energy, wherein theelectromagnetic energy includes at least two resonant maxima and thefirst energy feed is positioned at a first maximum of the at least tworesonant maxima and the bulb is positioned at a second maximum of the atleast two resonant maxima.
 34. The lamp of claim 31, further comprisingfirst and second energy feeds coupled to the waveguide for receiving theelectromagnetic energy, wherein the electromagnetic field includes atleast one resonant maxima and the bulb and the first energy feed areproximate to one of the at least one resonant maximum.
 35. The lamp ofclaim 31, further comprising the energy source and a feedback mechanismcoupled between the waveguide and the energy source, wherein thefeedback mechanism samples the electromagnetic field within thewaveguide, transmits the sampled field to the energy source, and theenergy source adjusts its delivery of electromagnetic energy to maximizethe electromagnetic field detected by the feedback mechanism.
 36. Thelamp of claim 35, further comprising a first energy feed coupled betweenthe energy source and the waveguide, wherein the electromagnetic energyincludes at least one resonant maximum and the first energy feed ispositioned approximately at a maximum of the at least one resonantmaximum and the bulb is positioned approximately at a maximum of the atleast one resonant maximum.
 37. The lamp of claim 1 further comprisingthe energy source.
 38. A lamp comprising: a waveguide comprising adielectric material and being thermally isolated from and configured toreceive electromagnetic energy from an energy source, said waveguidehaving a protrusion on a first side defining a bulb cavity and anelectrically and thermally conductive outer coating on an outer surfaceof the waveguide except the surface defining the protrusion; a bulbcontaining a gas-fill that produces light when receiving theelectromagnetic energy, said bulb being at least in part disposed in thebulb cavity and comprising: (a) a window, the window being substantiallytransparent to the light, and (b) an outer wall, the outer wall beinghermetically coupled with the window, shaped to direct the light towardsthe window, and having a thermal expansion coefficient approximatelyequal to the thermal expansion coefficient of the window, wherein thewindow and the outer wall define an envelope containing the gas-fill;and (c) a heat sink coupled to the outer surface of the waveguide. 39.The lamp of claim 38, wherein the electromagnetic energy resonateswithin the waveguide and comprises at least one resonant maximum, andwherein the bulb cavity and an input of the electromagnetic energy tothe waveguide are proximate to the at least one resonant maximum.
 40. Alamp comprising: first and second energy feeds for receivingelectromagnetic energy from an energy source; a waveguide having a bodycomprising a dielectric material, said waveguide being coupled to andfor receiving electromagnetic energy from the first energy feed and thesecond energy feed, having a bulb cavity, and an electrically andthermally conductive coating on the surfaces of the body except thesurfaces defining the cavity; a bulb containing a gas-fill, said bulbbeing disposed in the bulb cavity and comprising a window, the windowbeing substantially transparent to emitted light, and an outer wall, theouter wall being hermetically coupled with the window, shaped to directthe light towards the window, and having a thermal expansion coefficientapproximately equal to the thermal expansion coefficient of the window,wherein the window and the outer wall define an envelope of the bulb tocontain the gas-fill; and a heat sink coupled to the surface of thewaveguide.
 41. The lamp of claim 40, wherein the waveguide is configuredto contain resonant electromagnetic energy that comprises at least threeresonant maxima, the first energy feed being proximate to a firstresonant maximum, the second energy feed being proximate to a secondresonant maximum, and the cavity being proximate to a third resonantmaximum.
 42. A lamp comprising: a high frequency electromagnetic energysource having an output port and a feedback port; an energy feed coupledto the output port to receive electromagnetic energy from the energysource; a waveguide having a body comprising a dielectric material, saidwaveguide being coupled to and receiving electromagnetic energy from theenergy feed, having a bulb cavity in the body and a reflective outercoating; a feedback mechanism coupled between the feedback port and thewaveguide, the feedback mechanism for sampling the electromagneticenergy within the waveguide and for communicating amplitude and phase ofthe electromagnetic energy to the energy source, the energy sourceadjusting its output of electromagnetic energy to maximize theelectromagnetic energy detected by the feedback mechanism; a bulbcontaining a gas-fill that produces light when excited by theelectromagnetic energy, said bulb being disposed in the cavity; a heatsink coupled to a side of the waveguide.
 43. The lamp of claim 42,wherein the electromagnetic energy within the waveguide comprises atleast one resonant maximum, the energy feed being positioned at one ofthe at least one resonant maximum, the feedback mechanism beingpositioned to sample the resonant field, and the bulb cavity beingpositioned at one of the at least one resonant maximum.
 44. A lampcomprising: at least one energy feed for receiving electromagneticenergy from an energy source; a waveguide comprising a dielectricmaterial and coupled to the at least one energy feed for receivingelectromagnetic energy, said waveguide having a plurality of separatecavities, and an electrically and thermally conductive outer coatingdeposited on the outer surfaces of the dielectric except the surfacescomprising the plurality of bulb cavities; a plurality of bulbscontaining a noble gas and a light emitter that outputs light whenexcited by the electromagnetic energy, wherein each of the plurality ofbulbs is disposed in one of the plurality of bulb cavities and comprisesa window, the window being transparent to the light, and an inner wallshaped to direct the light towards the window and having a thermalexpansion coefficient approximately equal to the thermal expansioncoefficient of the window, the inner wall being hermetically coupled tothe window, the window and the interior wall thereby defining anenvelope in which the material is contained; and a plurality of heatsinks coupled to all sides of the waveguide, said plurality of heatsinks positioned to dissipate heat from the waveguide.
 45. The lamp ofclaim 44, wherein the electromagnetic energy is resonant within thewaveguide and comprises a plurality of energy maxima, the at least oneenergy feed being positioned approximately at at least one of theplurality of energy maxima.
 46. A lamp comprising: an electromagneticenergy source; an energy feed coupled to and receiving electromagneticenergy from the energy source; a dielectric waveguide thermally isolatedfrom the energy source and coupled to and receiving electromagneticenergy from the energy feed, said waveguide having a cavity and anelectrically and thermally conductive outer coating the outer surface ofthe dielectric material except the surface defining the cavity; athermal isolation layer lining the cavity; a bulb containing a materialthat produces light when excited by the electromagnetic energy, saidbulb being disposed in the cavity, with the thermal isolation layerseparating the bulb from the waveguide, and comprising a window, thewindow being transparent to the light, and an inner wall, the inner wallbeing hermetically coupled to the window shaped to direct the lighttowards the window, and having a thermal expansion coefficientapproximately equal to the thermal expansion coefficient of the window,the window and the inner wall defining an envelope in which the materialis contained; and a heat sink coupled to an outer surface of thewaveguide.
 47. The lamp of claim 46, wherein the electromagnetic energyresonates within the waveguide and comprises at least one resonantmaximum, the energy feed and the bulb cavity being proximate to the atleast one resonant maximum.
 48. The lamp of claim 46, wherein thethermal isolation layer comprises an evacuated space.
 49. The lamp ofclaim 46, wherein the thermal isolation layer comprises a seconddielectric material.
 50. A method for producing light comprising thesteps of: (a) generating electromagnetic energy; (b) directing theelectromagnetic energy into a dielectric waveguide having a cavity; (c)directing the electromagnetic energy into an envelope defined by thecavity and a window, the envelope containing a gas-fill; and (d)exciting the gas-fill into producing light.
 51. The method of claim 50further comprising the step of directing the produced light through thewindow.
 52. The method of claim 50, further comprising the step ofdissipating the heat generated by the plasma through the outer surfaceof the waveguide.
 53. The method of claim 50, comprising the steps of:(e) sampling the levels of electromagnetic energy within the waveguide,and (f) adjusting the frequency of the electromagnetic energy generateduntil the sampled electromagnetic energy is at a maximum.
 54. The methodof claim 50, further comprising the step of generating electromagneticresonance within the waveguide.